Hamilton will be gunning for his second straight victory and fourth overall at Circuit Gilles Villeneuve as he starts on the front row beside pole position holder and Formula One championship leader Sebastian Vettel on Sunday afternoon.
Cars were slipping and sliding on a mostly wet track through three practices and the qualifying session, and there could be more of that if track conditions do not improve.
"I was told it was going to be dry," Hamilton said Saturday. "But as Sebastian says, it's changing and it's going to be good fun.
"It should be interesting, as it's been for the last few years. It should be tricky, but I hope it's dry."
The constantly changing conditions saw Finnish rookie Valtteri Bottas, who has yet to earn an F1 point, place his Williams a surprising third on the grid.
The Canadian race is known for being unpredictable even in the best of weather, as drivers struggle to manage their brakes and tires on a slippery, high-speed track with long straightaways leading into nasty chicanes and one hairpin turn.
There are also concrete walls hugging much of the street-style track that have claimed many top drivers over the years.
Hamilton, who moved from the McLaren team to Mercedes this year, has avoided pitfalls enough to win in 2007, 2010 and last year. If he wins again, he will move into second place alone for most Canadian GP victories behind German great Michael Schumacher's seven.
"It's good to be on the front row," the British driver said. "That brings up the chances a little bit, but it's going to be a tough race.
"Obviously, Sebastian's very quick and the Ferraris were very quick on their long (practice) runs, so I guess it really depends on the conditions. At the end of the day, we've got to do the job. But as you know, I love it here. We had a great crowd even though the weather wasn't so great. I'm sure there will be even more people (Sunday), so let's hope we can put on a good show for them."
Forecasts are for rain overnight with mostly sunny, warm weather, although with a 60 per cent chance of a light sprinkle of rain. The crowd will likely top 100,000.
"In terms of grey hairs for us, it would probably be a bit nicer if it's a bit more predictable," said Vettel's Red Bull teammate Mark Webber.
Overnight rain had slowed to occasional trickles for qualifying, but it still had cars slipping onto the grass or, in the case of Ferrari driver Felipe Massa, taking a long, sideways slide into a barrier.
Vettel managed to post the quickest lap on the 4.361-kilometre track in one minute 22.318 seconds to take pole position for a third straight year. The 39th pole of his career was his third this season, and ended Mercedes' run of four in a row.
The 25-year-old German is the three-time defending F1 champion, but has never won in Canada. He was second in 2011, and finished fourth twice.
"I think we had strong pace, so no matter the conditions, we should be in good shape," he said. "It could be an interesting race with mixed conditions. So, a long one. Seventy laps here and a lot of things that can happen on every single lap."
Timing looked to be everything in qualifying. There were yellow caution flags and even a red flag that stopped the session for a few minutes when Massa, who crashed twice in Monaco two weeks ago, found a barrier again going into Turn 3.
Conditions were such that Bottas was not about to crow about placing third. His teammate Pastor Maldonado was 13th.
"We didn't expect to be in the first or second row," said Bottas. "Our whole team was hoping for rain because there's always a chance that if you get the timing and everything right you could finish a bit higher.
"But if it's dry (in the race), for sure it's going to be difficult. Today was a lot about the timing, getting a good lap just at the right time when the track was in the best conditions. That's why there can be 10 places between teammates."
Hamilton's teammate Nico Rosberg, coming off a win in Monaco, saw his run of three straight poles end as he placed fourth, just ahead of Webber and Ferrari veteran Fernando Alonso.
It was a good day for the Toro Rossi squad, as Jean-Eric Vergne was seventh and Daniel Ricciardo will start 11th. Ricciardo would have had an even better position, but he was penalized two grid spots for not starting from the proper lane.
It was not a good day for Kimi Raikkonen of Lotus, who will start in 10th after he was also dropped two spots due to the same penalty. Raikkonen sits second to Vettel in driver standings.
McLaren's Jenson Button, the 2011 Canadian GP winner, placed 14th.
Vettel and Alonso have each won twice this year. The Spaniard hopes to take a shot at a third title, and first with Ferrari, to go with his championships in 2005 and 2006 with Renault.
"If we could finish in front of our rivals in the championship it would be great," said Alonso. "Kimi is (behind), so it's a good opportunity.
"Sebastian will be more difficult. We need a very good race."
And there is Hamilton, who thrives on the track that has given rivals so much grief. Even he was not pleased at a minor slip on his last qualifying lap.
"The pole was definitely there, so it's a bit unfortunate, but that's the way it goes," he said.
The pace was slowed considerably by the weather. Vettel won the pole last year in 1:13.784.
The morning practice was cut from one hour to 30 minutes as a barrier needed repair from a crash during a support race Saturday morning. That ate up some valuable information-gathering time for the 11 teams ahead of the early-afternoon qualifying session.
Race stewards announced that Webber and Bottas were reprimanded for ignoring a yellow flag during practice on Friday. The flag went up after a Maldonado crash.Suggest a correction